In early 1924, James Couzens was a Republican Senator from Michigan and reportedly the richest member of Congress. Andrew Mellon was beginning his fourth year as Secretary of the Treasury — a service that would eventually span 11 years under three Republican Administrations — and one of the wealthiest persons in the entire country. This article describes how a feud between these two men, an ensuing investigation led by Couzens of the Bureau of Internal Revenue (BIR) (predecessor to the modern-day IRS), and a tax case against Couzens that was described as the “greatest tax suit in the history of the world,” helped lead to creation of the U.S. Joint Committee on Taxation (JCT) and its staff. Drawing upon many previously unreported key details, the article reveals a fascinating tale filled with political intrigue, backstabbing (real or imagined), and unintended consequences. The article shows how the events antagonized Congress’s relationship with the executive branch, but improved cooperation between the House and Senate, and both were instrumental in the JCT’s creation. The article also provides insight on the unique role the JCT has played in Congress for over 85 years. Finally, the article explains how creation of the JCT became entangled with two of the most contentious tax issues of the day — the publicity of tax return information and the depletion allowance for oil and gas production — and played a role in changing the law in both areas.